The present invention relates to an optical disc apparatus that is capable of using a plurality of kinds of optical discs whose cover layers are different in thickness.
The optical disc apparatus includes a turntable that rotates the optical disc mounted thereon and an optical pick-up that reproduces recorded information from the optical disc and/or records information onto the optical disc. The optical pick-up is provided with a laser source such as a semiconductor laser, an objective lens that converges the laser beam emitted from the laser source and a light receiving portion that receives a beam reflected by the optical disc. The optical pick-up is movable in a radial direction of the optical disc.
The optical disc includes an information layer on which digital information is recorded, and a transparent cover layer that covers the information layer.
A laser beam from the laser source is converged by the objective lens and forms a beam spot on the information layer through the cover layer. The light receiving portion receives the reflected beam from the optical disc to produce a focusing error signal, a tracking error-signal and a reproducing signal.
There are a few types of the optical discs. A CD (compact disc) or a CD-R (CD recordable) has the cover layer whose thickness is 1.2 mm, and the thickness of the cover layer of a DVD (digital versatile disc) is 0.6 mm. Recording density of a DVD is higher than that of a CD or a CD-R.
Such a difference of thickness of the cover layer changes the relative position of the information layer with respect to the turntable, i.e., the distance between the optical pick-up and the information layer. Namely, the thicker the cover layer is, the greater the distance to the information layer from the optical pick-up is. For example, the optical pick-up is required to move the beam spot away from the optical pick-up by 0.6 mm in the cover layer, which is equivalent to 0.4 mm in air, when the DVD is replaced with a CD or a CD-R.
In order to be adapted for a plurality of kinds of optical discs whose cover layers are different in thickness, a conventional optical disc apparatus is equipped with a plurality of optical pick-ups to be selectably employed in accordance with the thickness of the cover layer. As the other conventional optical disc apparatus, there has been one which is equipped with a plurality of objective lenses to be selectably employed. However, the conventional optical disc apparatuses that include a plurality of optical pick-ups or objective lenses increase the total size of the apparatus. Therefore, it is preferable to use a common optical pick-up and a common objective lens to make the apparatus compact.
When a common optical pick-up and a common objective lens are used, there are two ways to move the beam spot in an optical axis direction according to the change of the thickness of the cover layer. The first way is to move the optical pick-up in the optical axis direction. The second way is to change the distance between the light source and the objective lens. However, the first way has such a disadvantage that a large moving mechanism is required for focusing.
The second way is effective for an optical disc apparatus that employs a plurality of separate semiconductor lasers elements. A compatible optical disc apparatus for CD, CD-R and DVD is provided with a first semiconductor laser whose emission wavelength is 635 nm through 660 nm and a second semiconductor laser whose emission wavelength is 780 nm through 830 nm.
The reasons why the optical disc apparatus needs the first and second semiconductor lasers are differences in recording density and reflection characteristics of the optical discs. The recording density of a DVD is higher than that of a CD, which requires to form a smaller beam spot on a DVD than the beam spot formed on a CD. Since the diameter of the beam spot becomes smaller as the wavelength of the laser beam decreases, the optical pick-up for a DVD requires the laser source whose emission wavelength is 635 through 660 nm. On the other hand, the reflection characteristics of a CD-R requires the laser source whose emission wavelength is 780 nm through 830 nm. When the optical disc apparatus includes a pair of semiconductor laser elements, the semiconductor laser elements can be positioned such that the distances between the semiconductor laser elements and the optical disc are different to each other.
However, the second way is not applicable to an optical disc apparatus that employs a module where semiconductor lasers and light receiving elements are mounted on a single circuit board. Such a module is disclosed in pages 29 and 30 of xe2x80x9cNikkei Electronics 1999.6.26 (No. 746)xe2x80x9d. In the module, two active layers having different characteristics are formed on a single chip that emit laser beams at 650 nm and 780 nm, respectively. The active layer has a point from which the laser beam is emitted. This point is referred to as an emission point. The module has a pair of the emission points.
When a plurality of emission points are formed in close proximity to each other as described above, since the distances between the respective emission point and the surface of the cover layer is identical even if the thickness of the cover layer varies, the second way is not applicable.
It is therefore an object of the present invention to provide an optical system of an optical disc apparatus capable of adapting to a plurality of kinds of optical discs whose cover layers are different in thickness without moving the whole system of the optical pick-up even when the emission points are formed in close proximity to each other.
For the above object, according to the present invention, there is provided an improved optical disc apparatus that is adapted for a plurality of kinds of optical discs whose cover layers are different in thickness, which includes a light source portion that has a plurality of emission points in close proximity to each other, an objective lens for converging divergent laser beam from the light source portion onto an information layer of the optical disc through the cover layer thereof, a light receiving portion that receives the reflected light beam from the optical disc to produce signals, and a focusing mechanism that moves the objective lens. The emission points emit divergent laser beams at different wavelengths, respectively, and the emission points are switched in accordance with the thickness of the cover layer. The distances between the respective emission points and the surfaces of the cover layers are constant regardless of the thickness of the cover layer.
Further, the focusing mechanism moves the objective lens such that the distance between the objective lens and the optical disc increases as the thickness of the cover layer decreases. That is, since the information layer is positioned behind the cover layer, the information layer becomes farther from the light source portion as the thickness of the cover layer increases. Therefore, when the distance between the objective lens and the optical disc changes as described above, the beam spot follows the information layer.
With this construction, the beam spot moves in the optical axis direction by moving the objective lens in accordance with the thickness of the cover layer, which brings the beam spot to be coincident with the information layer of the optical disc.
Although a paraxial beam spot moves as the objective lens is moved, the change of the thickness of the cover layer changes spherical aberration. If the optical disc apparatus only moves the objective lens when the disc is replaced, wave front aberration of the laser beam is deteriorated, thereby the diameter of the beam spot increases, which prevents the optical disc apparatus from reproducing the recorded information from the optical disc.
Therefore, it is preferable to give wavelength dependence to the objective lens such that spherical aberration varies as wavelength of incident laser beam varies, thereby canceling the variation of the spherical aberration caused by the change of the thickness of the cover layer.
The wavelength dependence of the spherical aberration can be achieved by adding a diffractive lens structure formed on either lens surface of the objective lens. Since the recording density of a DVD having the thinner cover layer is higher than a CD or CD-R having the thicker cover layer, a first wavelength of the laser beam for the optical disc having a thin cover layer should be smaller than a second wavelength of the laser beam for the optical disc having a thick cover layer. Therefore, the diffractive lens structure should have the wavelength dependence such that a predetermined order diffraction light at the first wavelength forms an appropriate wavefront for the optical disc having the thin cover layer and the same order diffraction light at the second wavelength forms an appropriate wavefront for the optical disc having the thick cover layer.
Further, since the spherical aberration varies in an overcorrected direction as the thickness of the cover layer increases, the diffractive lens structure is preferable to have the wavelength dependence such that the spherical aberration varies in an undercorrected direction as the wavelength of the incident light increases. An additional optical path length added by a diffractive lens structure is expressed by the following optical path difference function "PHgr"(h):
"PHgr"(h)=(P2h2+P4h4+P6h6+ . . . )xc3x97xcex
where P2, P4 and P6 are diffractive coefficients of second, fourth and sixth orders, h is the height from the optical axis and xcex is wavelength of incident light.
The objective lens embodying the invention may satisfy the following condition (1);
xe2x88x9215 less than "PHgr"(h45)/xcexxe2x88x92P2xc3x97(h45)2 less than xe2x88x9210xe2x80x83xe2x80x83(1)
where h45 is the h where a light ray whose NA is 0.45 intersects the diffractive lens structure.
Still further, it is preferable that the focal length of the objective lens at the first wavelength used for the optical disc having the thin cover layer is shorter than the focal length of the objective lens at the second wavelength used for the optical disc having the thick cover layer.
Yet further, it is preferable that the diffractive lens structure has a negative lens power, i.e., the second order coefficient P2 of the optical path difference function has a positive value. The lens power is defined as a power that converges or diverges the light beam. The lens power of the objective lens is determined as a total sum of the lens power of the refractive lens and the lens power of the diffractive lens structure. Since the objective lens converges the divergent laser beam onto the optical disc, the total lens power is positive. And thus the refractive lens has a large positive power, i.e., a positive short focal length. However, if the total lens power becomes too large, the moving amount of the objective lens when the optical disc is replaced becomes too large, which increases burden on the moving mechanism. When the diffractive lens structure has the negative lens power as described above, the total lens power becomes smaller, which reduces the moving amount of the objective lens.
The light source portion may be provided with two emission points on one chip. Further, it is preferable that the light receiving portion is mounted on the board on which the light source portion is mounted for compact design.